NGSP
The National Glycohemoglobin Standardization Program certifies A1C assay methods against a reference method. Its certification criteria are the reason a small difference between two A1C results may be the test rather than a change in you.
Example
Certification permits a certified method to sit roughly half a percentage point from the reference method, so a 5.6% and a 5.8% from the same person may not represent any real difference.